Voltage regulators are commonly used in conjunction with additional electronic components or circuitry to provide a source of voltage at a desired level based on an input voltage from a power supply. In general, voltage regulators are intended to provide a relatively constant output voltage and typically have circuitry that continuously maintain the output voltage at a desired value, regardless of fluctuations in load current or input voltage, provided that the fluctuations are within specified operating ranges.
During start-up of a conventional voltage regulator, the voltage regulator draws current from the power supply. A slow ramp-up of output voltage by the voltage regulator (commonly known as “softstart”) is common practice to limit the impact of current demands from the voltage regulator on the power supply. With softstart, the voltage regulator tends to “pull-up” to the desired output voltage by drawing a less demanding amount of current from the power supply. One known voltage regulator is a switching, direct current-to-direct current (DC/DC) converter having a power stage producing the output voltage and a control loop that regulates the output voltage at the desired value. The control loop has an input for a reference voltage that is used to establish a base value for the output voltage. For this DC/DC converter, softstart may be implemented by ramping the reference voltage of the control loop.
A conventional reference ramp generator 20 for ramping the reference voltage of the DC/DC converter is shown in FIG. 1. The reference ramp generator 20 outputs a voltage, Vramp, and includes a capacitor 24 having a capacitance (Cap), a first terminal coupled to a voltage controlled current source 22 and a second terminal coupled to a reference potential (e.g., ground). Current source 22 generates a reference current (Iref), based on a supply voltage, Vdd. The ramp voltage (Vramp) ramps from the reference potential to the desired reference voltage at a rate, dV/dt, generally depending on the size of capacitor 24 and the value of Iref. For the reference ramp 20, the rate of change of Vramp is governed by the equationdV=(Iref/Cap)×dt.
FIG. 2 is a graph illustrating the voltage output (Vramp) of the reference ramp generator 20 shown in FIG. 1 as a function of time. In integrated circuits (IC) or monolithic devices, the ramp time from the reference potential to the desired reference voltage is generally a function of Iref and Cap as described above. For example, IC devices may have variable characteristics introduced by process control variations and leakage. A minimum reference current (e.g., 1 μA) is typically utilized to maintain accuracy. Additionally, since cost and size limitations generally limit the capacitance to, for example, less than 100 pF, the ramp time is typically limited to a period that is substantially less than one ms. For example, using a bandgap reference voltage of 1.25V, a current of 1 μA and a capacitance of 100 pF, the ramp time of the reference ramp 20 shown in FIG. 1, is:dt=(100 pF/1 μA)×1.25 V=0.125 ms.
Accordingly, a reference ramp having a longer softstart times than conventional reference ramps is desired for on-chip devices to further reduce impact on the power supply during start-up. In addition, a voltage regulator circuit is desired having a longer softstart time without a substantial increase in the size and cost of the circuit. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.